In recent days, with the progress of aging society, the number of patients in need of support and care has quickly increased. About 20 to 30% of these patients have disorders in functions of motors (skeletons such as joint and spinal, nerve for moving skeletons, muscle and ligament, etc.). In particular, loss of walking ability leads to an increase in the risk of other diseases such as dementia and visceral disease.
In the case of losing walking ability due to external injuries of a hip joint (bone fracture, etc.) or diseases of a hip joint (hip osteoarthritis, rheumatoid arthritis, etc.), one choice may be artificial hip joint replacement arthroplasty. The artificial hip joint replacement arthroplasty is an operation of replacing a hip joint with function disorder by an artificial joint replacement. The artificial hip joint replacement arthroplasty enables patients to not only recover walking ability, but also to get relief from pain of a hip joint.
Lifetime of various artificial joint replacements including an artificial hip joint replacement is usually said to be about 15 to 20 years. Therefore, patients may undergo a revision surgery of an artificial joint replacement (the revision surgery for the artificial joint replacement) 15 to 20 years after they underwent the artificial hip joint replacement arthroplasty. In order to avoid the revision artificial joint replacement arthroplasty as much as possible, an artificial joint replacement with longer lifetime is desired.
One reason for needing the revision surgery of the artificial joint replacement is loosening of an artificial joint replacement. For example, in a lot of artificial hip joint replacements, a liner made of polyethylene (PE), and a femoral head made of metal or ceramic are used. When the artificial joint replacement is operated, the liner made of a soft material is worn away by the femoral head made of a hard material to generate submicron-sized wear debris of PE. When macrophages eat the wear debris generated in the living body, macrophages accelerate formation of osteoclasts. As a result, osteolysis due to osteoclasts occurs in the bone surrounding the implant to form a gap between the bone and the artificial joint replacement, leading to loosening of the artificial joint replacement. In this way, loosening of the artificial joint replacement occurs due to the vital reaction against a foreign substance called “wear debris of PE”.
Therefore, “reduction in generation of wear debris of PE (improvement in wear-resistance)” is effective so as to suppress loosening of the artificial joint replacement.
It has been known that a liner made of crosslinked PE (CLPE) is used so as to improve wear-resistance (for example, Patent Document 1). CLPE is obtained by irradiating common PE with high energy beams (a gamma-ray, an electron beam, etc.) to cause the crosslinking reaction. CLPE can reduce the generation of wear debris from the liner since it is excellent in wear-resistance as compared with the common PE.
However, when a free radical is generated in PE by irradiation with high energy beams, the molecular chain of PE may be cut by the free radical, leading to a decrease in mechanical strength of the liner. The liner disposed in the living body over a long period of time may cause a decrease in mechanical strength of the liner as a result of cutting of the molecular chain of PE due to oxidation. The decrease in mechanical strength of the liner is not desirable since it causes deterioration of load supportability and wear-resistance of the liner.
There has been made an attempt to produce a liner using PE including a radical scavenger (for example, vitamin A, vitamin C, and vitamin E) so as to inactivate a free radical in PE and to enhance oxidation resistance of PE (for example, Patent Document 2).
There has been known, as another attempt to improve wear-resistance, coating of a surface of the liner with a highly lubricating polymer film (for example, Patent Documents 3 to 5). There has been used, as the polymer film, a 2-methacryloyloxyethyl phosphorylcholine (MPC) film which exhibits extremely highly lubricity under wet environment.
Patent Document 1: JP 2000-514481 W
Patent Document 2: JP 4256096 B1
Patent Document 3: JP 2003-310649 A
Patent Document 4: JP 4963838 B1
Patent Document 5: JP 5028080 B1